1. Field of the Invention
The present invention relates generally to electric motor/generators and, more specifically, to a permanent magnet, axial field motor/generator.
2. Description of the Related Art
An electric motor, which is a machine for converting current into motion, typically includes a rotor that rotates within a stator in response to a magnetic field. In a permanent magnet motor, the rotor or stator, typically the stator, includes one or more permanent magnets that generate a magnetic field. Permanent magnets may be made of ferrous metals or ferroceramic materials. Because the machine may also be used to convert motion into electric current, the machine is often referred to in the art as a motor/generator or a dynamo. Therefore, although the term "motor" is used herein for convenience, it should be understood that the same machine may be used as a generator.
The rotor or stator of a permanent magnet motor, typically the rotor, includes conductors, such as copper wire, wound around a form. These windings typically have numerous turns of the wire in order to maximize the magnitude of the magnetic field.
In certain permanent magnet motors, the stator includes a metal casing that holds two or more magnets and completes the magnetic circuit between them. The casing typically comprises metal plates or laminations to minimize eddy currents. Increasing the amount of metal in the casing lowers the reluctance of the circuit, thereby increasing the proportion of magnetic flux in the gaps between the magnet poles through which the rotor windings move.
Common motor magnet materials include iron, an aluminum-nickel-cobalt alloy known as Alnico, and rare-earth materials, such as a samarium-cobalt alloy. These materials provide a strong magnetic field but are quite heavy.
Motor magnets commonly must be magnetized in-place, i.e., after they have been assembled into the metal motor casing, to minimize demagnetization upon assembly. If a magnet is removed from the casing (or inserted into it), the act of removing (or inserting) it generally demagnetizes it to some extent. Replacing the magnet would result in diminished performance. To minimize demagnetization, a metal "keeper" may temporarily be attached to a magnet prior to mounting it in a motor or other device.
Practitioners in the art have developed axial field motors having magnets disposed on a rotor with their fields aligned parallel to the axis of rotation of the motor. Axial field motors do not require heavy metal casings to contain the field. Practitioners in the art have developed small axial field motors that include economical ferroceramic magnets. Unlike rare earth magnets, ferroceramic magnets can readily be magnetized into multiple poles because they have a relative permeability (".mu.") on the order of 1. (Permeability is defined as the ratio between the magnetic field density (B) of a material to its magnetic field intensity (H). Air, by definition, has a relative permeability of 1.) In certain such motors, each rotor disc is magnetized into multiple sector-shaped poles. Each sector has a polarity opposite that of an adjacent sector, and each sector is polarized through the thickness of the disc. The rotor is disposed adjacent to the stator on a common axis. The stator of such a motor typically consists of multiple toroidal windings. The magnetic field through which the windings pass is concentrated between two adjacent sectors of the same disc.
Motors having ferroceramic magnets produce lower torque than motors having magnets made of high-permeability materials, such as iron, Alnico and rare earth materials, because ferroceramic magnets exhibit a lower flux density. To obtain an increase in torque, the rotor may have two disc magnets, one on each side of the stator. Half of each toroidal winding of the stator thus passes through the magnetic field generated by one magnet of the rotor, and the other half of the winding passes through the magnetic field generated by the other rotor magnet. Nevertheless, the density of the flux through which each winding half moves is limited to that produced by the disc magnet to which it is closest. The use of multiple pole ferroceramic magnets is therefore largely confined to small, low-torque motors, such as stepper-type motors used in disk drives.
It would be desirable to provide an economical motor that has a high power-to-weight or efficiency ratio. These needs are clearly expressed in the art and are satisfied by the present invention in the manner described below.